The present invention relates to electrical circuit protection devices such as overload relays, and more particularly to an improved and highly flexible electronic control system therefor.
Circuit protection devices such as circuit breakers, relays, contactors and the like are commonly used for disconnecting electrical circuits upon the detection of undesired currents. In addition to breaking the circuits in which the currents flow, other functions may be provided such as actuating alarms and safety devices, or the control of other apparatus in response to a sensed current characteristic. While in principle the opening of an electrical circuit in response to undesirably high currents is a simple procedure, in practice the operation of such protective devices is highly complex owing to the various, often conflicting requirements of electrical systems.
For instance, while it is necessary to protect electric motors from high currents which could damage or destroy the windings, in order to start a motor under load a high initial current is required. Also, during the operation of various electrical equipment, for instance under changing loads, high current flow must be tolerated for short periods of time. Further a single "threshold" for current flow cannot be assigned since a small overcurrent condition can be tolerated far longer than a high overcurrent condition. For these reasons industrial relays and contactors are commonly provided with complex control mechanisms which make use of two or more different current-responsive stages in an attempt to "tailor" the tripping characteristics of the device to a desired application.
In principle it is known that more sophisticated control systems can be designed to replace the present electromechanical, magnetic, and thermal controls. If cost were not a consideration a number of approaches could be implemented for deriving a measure of current flow and converting it into appropriate digital form. However for small, inexpensive control devices such as overload relays conventional means of deriving and processing signals have proven prohibitively expensive and overly complex.
It is therefore an object of the present invention to provide an improved control apparatus for circuit interrupters of the overload relay and contactor type.
Another object is to provide an interrupter control which can be utilized to monitor single or plural phase electric power sources.
Another object is to provide an inexpensive solid-state control which utilizes relatively simple digital signals to detect imbalances in polyphase current flow.
It is a further object of the invention to provide a low cost control means for an overload relay or similar circuit interrupter which can monitor plural phases without the need for signal multiplexing or utilizing redundant signal processing paths.
Yet another object is to provide a method for controlling system response to an overcurrent condition as a function of current imbalance.